PROJECT SUMMARY/ABSTRACT Callous-unemotional behaviors (CU), characterized by a lack of guilt, empathy, and affect, have significant negative implications. CU is predictive of conduct problems and antisocial outcomes in adulthood, and indexes a particularly aggressive subset of adolescents with these severe behavior problems, placing a large burden on families and society. Even in individuals without severe behavior problems, CU is related to problematic behaviors such as bullying and aggression. Early childhood and adolescence are critical periods in the development of CU. In early childhood, behaviors relevant to CU (e.g., guilt, empathy) come online and are most receptive to intervention. In adolescence, CU becomes more refined and is implicated in aggression, delinquency, prosociality, and conduct disorder. Research on early CU behaviors has become more prevalent in recent years, but the long-term developmental significance of these early socio-emotional behaviors is unknown. This will be the first study to examine the links between early and adolescent CU and related behaviors. Understanding the long-term stability and predictive validity of early CU into adolescence, as well as the genetic and environmental sources of these potential links across age, is crucial in identifying young children at genetic and environmental risk for later maladaptive problems in adolescence that are costly to families and society. The proposed research is an online longitudinal follow-up of 314 twin pairs previously assessed at ages 2 and 3 years. To allow a more nuanced understanding of CU, and to ensure scientific rigor, our assessments will include self- and parent-reports and a behavioral game to address our three aims: (1) Investigate the developmental significance of early CU and related behaviors on later adolescent CU and socio-emotional behaviors (e.g., does early CU predict later outcomes?); (2) Explore the genetic and environmental contributions to longitudinal associations that emerge for CU and related behaviors; (3) Examine the concurrent and longitudinal relations between CU and prosociality in adolescence using parent- and self- reports and a behavioral game of cooperation (as aspect of prosociality). The potential benefits of this project to society are high. CU in adolescence is implicated in various costly behaviors and is difficult to treat. Thus, identifying the extent to which early behaviors predict adolescent CU and other maladaptive behaviors, and the mechanisms underlying these associations, is necessary in aiding early intervention efforts.